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PLATETECTONICS Or Are we dancing on an eggshell? Is there a magma ocean below? …and much more…

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Presentation on theme: "PLATETECTONICS Or Are we dancing on an eggshell? Is there a magma ocean below? …and much more…"— Presentation transcript:

1 PLATETECTONICS Or Are we dancing on an eggshell? Is there a magma ocean below? …and much more…

2 PLATETECTONICS

3 What causes plate movement? Heat transfer by mantle convection!

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5 Earth’s mantle: peridotite!!! geotherm

6 olivine

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8 Olivine (cut gems)

9 Olivine: thin section

10 Olivine with rims altered to iddingsite

11 Pyroxenes: Orthopyroxenes: (Mg,Fe 2+ ) 2 [SiO 6 ] Clinopyroxenes: (Ca,Na,Li)(Mg,Fe 2+,Fe 3+,Mn,Al,Ti)[(Si,Al) 2 O 6 ]

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13 Clinopyroxene: thin section

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15 Plagioclase: albite

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17 Plagioclase: thin section

18 Olivine (coloured) in plagioclase (grey)

19 Garnet: (Ca,Mg,Fe,Mn) 3 (Al,Fe,Cr) 3 [SiO 4 ] 3

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21 Garnet (cut gem)

22 Garnet: thin section (polarized light)

23 Earth’s mantle: peridotite!!! geotherm

24 Earth’s geotherm is well-below the solidus! So why does magma form? i.e., why does the mantle melt?

25 geotherm

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30 …now back to the surface… moving oceanic and continental plates

31 plate movement in cm/yr

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33 earthquakes: at plateboundaries (in particular along subduction zones)

34 …creation of oceanic crust… SPREADING ZONES

35 mantle convection drives plate tectonics!

36 heat causes convection - heat transport by convection

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38 mid-oceanic ridges / spreading zones: “constructive” and “divergent” plate boundaries

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41 Structure of the oceanic crust

42 Recent (modern) pillow basalts at the seafloor off the US west-coast

43 Ancient pillow basalt

44 in a road cut black pillow rims are basaltic glass formed by quenching of hot lava during contact with cold seawater

45 hydrothermal vent: black smoker: hot, acidic, reducing (colour due to formation of black sulphide particles)

46 hydrothermal vent: white smoker: <200 o C (colour due to formation of silica and sulphate particles)

47 hydrothermal vents: black and white smokers

48 most recently discovered (!!!): new type of (cold) hydrothermal vent (heat produced by alteration of oceanic crust, white minerals are Ca carbonate)

49 Structure of the oceanic crust

50 Ophiolites: -pieces of oceanic crust thrusted (obducted) onto the edge of continental plates; - assemblage of mafic (basalt) and ultramafic (perodotite) lavas found in association with sedimentary rocks; -found in Cyprus, New Guinea, Newfoundland, California, Turkey, and Oman. S ophiolite in southeastern Oman probably formed in the Cretaceous not far from what is now the Persian Gulf. The rocks were later thrust (pushed uphill at a low angle) westward onto the Arabian shield.

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54 Ophiolites in the middle east

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56 marine sediment: ferrugineous radiolarian chert (biogenic quartz) igneous rock: pillow basalt

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58 sheeted dikes (basalt)

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60 layered gabbros (gabbro is the coarse-grained equivalent of a basalt)

61 close-up view of layered gabbro

62 Moho

63 Layered harzburgite (ultramafic mantle rock)

64 SUBDUCTION ZONES …and now for something completely different:

65 “destructive” and “converging” plate boundaries

66 Two types of destructive plate boundaries: - oceanic crust subducted under oceanic crust - oceanic crust subducted under continental crust [oceanic (basaltic) crust is denser than continental (granitic) crust]

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68 Mount Fuji, Japan

69 Mount Mayon, Philippines

70 stratovolcanoes in the Andes at the Chile-Bolivia border

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73 Andesite: dark pyroxene phenocrysts* in grey groundmass of plagioclase and minor pyroxene and olivine *large crystals that formed in the magma prior to eruption

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75 viscous andesite lava flows on the flanks of Colima stratovolcano, Mexico

76 channelled andesite lava flows at Lascar volcano, Chile

77 What causes island-arc volcanism? What does actually melt and how?

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81 !!!!!!

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83 slab pull causes convection in the mantle wedge back-arc spreading

84 back-arc spreading causes basin formation and volcanism

85 Sediments and plate tectonics

86 Chemical evolution of mantle and crust

87 bulk EarthEarth’s crust

88 Crustal growth throughout Earth’s history was episodic!

89 two models for recycling of Earth’s crust

90 subducted slab “stored” at 650km discontinuity subducted slab “stored” in the Lower Mantle

91 so much for PLATE TECTONICS…


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